1. Field of Invention
This invention relates to a heat exchange tube bundle having a uniformly densified structure. More particularly, this invention relates to such a bundle and method of manufacture in which dimples are provided at least at overlap regions of return bends so that resultant overlapping tubes can be packed with an increased density in which the circuit-to-circuit spacing between adjacent tubes is less than the projected cross-sectional area of the individual tubes.
2. Description of Related Art
Various heat transfer tube bundle systems are known. Condensers and closed circuit cooling towers typically include a bundle of numerous lengths of tubing in an array. The tubing may be in serpentine form or as a series of discrete tubes that run into a header section. The tubing contains a condensing vapor or a medium to be cooled, such as water. In the finished product, air and/or water is forced to flow over the external surfaces of the tubing.
Counterflow evaporative heat exchangers are shown and described in, for example, U.S. Pat. Nos. 3,132,190 and 3,265,372. Those heat exchangers comprise an upwardly extending conduit containing an array of tubes which form a coil assembly. A spray section is provided in the conduit above the coil assembly to spray water down over the tubes; and a fan is arranged to blow air into the conduit near the bottom thereof and up between the tubes in counterflow relationship to the downwardly flowing sprayed water. Heat from the fluid passing through the coil assembly tubes is transferred through the tube walls to the water sprayed down over the tubes; and the upwardly flowing air causes partial evaporation of some of the water and transfer of heat and mass from the water to the air. The thus heated and humidified air then flows upwardly and out from the system. The remaining water collects at the bottom of the conduit and is pumped back up and out through spray nozzles in recirculatory fashion.
There are other evaporative type heat exchangers in which the liquid and gas flow in the same direction over the coil assembly. Examples of these other devices, which are generally referred to as co-current flow heat exchangers, are shown in U.S. Pat. Nos. 2,752,124, 2,890,864, 2,919,559, 3,148,516 and 3,800,553.
The above are types of coil only heat exchangers. There are other types, such as coil/fill types that are provided with both an indirect evaporative heat exchanger section and a direct evaporative heat exchanger system. U.S. Pat. No. 5,435,382 is an example of such a heat exchanger.
Various different methodologies of heat transfer tube bundle designs have been tried in the above conventional systems. In earlier designs, coil assemblies of round tubing were packed into tight arrays to increase surface area. The number of circuits that could be packed into a serpentine tube bundle was limited by the diameter of the tubing. This was because the return bends overlapped each other and would thus touch when spaced close together.
Subsequent designs, such as U.S. Pat. No. 4,196,157, were directed to a sparsified heat transfer tube bundle in which the spacing was increased to allow more airflow between the tubes, higher internal film coefficient, and better wetting of the tubes in attempts to increase total heat transfer rates. Other designs such as those in U.S. Pat. Nos. 5,425,414 and 5,799,725 kept packing density high and used circular return bend systems, but provided elliptical tube sections in the straight sections in an attempt to increase airflow. Packing in such examples was again limited by the diameter of the circular return bend. German Patent Publication No. DE3,413,999C2 is directed to oval tubes and describes problems forming oval tubes into U-bends.
Some prior art designs attempted to increase capacity by xe2x80x9cpulling downxe2x80x9d the bundled tubing slightly, such as by compressed clamping of the entire bundle during assembly. While this has been found to allow for slightly tighter spacing for a given heat exchanger size (typically {fraction (1/64)}xe2x80x3 or so), such compression does not act uniformly on the tube bundle, but instead focuses compression forces on the endmost tubes. If the pull down is excessive, this results in a tube bundle with inconsistent flow properties, since the endmost tubes (uppermost and lowermost) may be disproportionately deformed so as to cause a flow or pressure problem at these circuits. For these reasons, xe2x80x9cpull downxe2x80x9d has typically been limited to no more than 2% of the return bend width. Thus, packing has been limited to a density that was typically less than 1.0, and possibly slightly greater than 1.0 (up to 1.02) through xe2x80x9cpull downxe2x80x9d. However, such increased density was not controllably uniform or precise.
There is a need for an improved heat exchanger tube bundle design and method of manufacture that can increase heat transfer surface area for a given heat exchanger size.
There also is a need for a heat exchanger tube bundle design that can increase bundle density. There is a particular need for a heat exchanger tube bundle design that increases bundle density uniformly, so that all circuits can maintain consistent functionality.
The invention allows for increased heat transfer surface area to be packed into the same space/size constraints of prior designs or, conversely, allows the same heat transfer surface area of the prior art to be provided in an enclosure that occupies less space. Either technique increases the heat transfer surface area/cost ratio. The invention also reduces pressure drop in the heat exchanger by providing more circuits over prior art designs.
The present invention achieves these objects in a novel manner. According to one aspect of the present invention, the number of tubes in the coil assembly of a heat exchanger is increased from that which would previously have been considered possible to provide maximum heat transfer surface area for a given heat exchanger size. The coil assembly is made up of arrays of substantially equally spaced apart tube segments located at different levels in the coil assembly. According to this aspect of the invention, the coil assembly is arranged to have individual circuits of an effective diameter D and a circuit-to-circuit spacing S that is less than D. When a non-circular cross section is used, the outside perimeter of the tube divided by pi is considered as the effective diameter D.
The invention may be practiced in most any type of heat exchanger where overlapping circuits of tubing are provided. Tubing may be continuous or discontinuous, such as such as straight tubing with separately fabricated return bends. Non-limiting examples include evaporatively cooled heat exchangers, air cooled heat exchangers, and shell and tube heat exchangers. The inventive coil assembly is particularly advantageous for use with serpentine tubing. Coil-only type heat exchangers may show improved performance properties since the inventive coil assembly allows more heat transfer surface area to be provided in the same space constraint. However, in certain applications there may be an adverse decreased airflow, since the flow path between the circuits is marginally decreased, which offsets some of the thermal advantage of more heat transfer surface area. The invention, however, is more preferably useful in coil/fill type heat exchangers because the increase in tube bundle density does not decrease overall unit air flow to the same degree that it may in a traditional coil only tube bundle.
The use of dimpling to locally reduce the outer dimensions of the tubing in the area of overlap is advantageous, since it has only a minimal increase in internal fluid pressure drop compared to compressing of the entire return bend. Moreover, dimples are easier to form than compression of an entire return bend, while having minimal, if any, effect on the structural characteristics of the tubing. Moreover, the stacking of adjacent tubing that nests in the dimple serves to reinforce the dimple area, reducing any such effect.
In embodiments of the invention, indentations or xe2x80x9cdimplesxe2x80x9d of predetermined dimensions, preferably having a depth of 2.5% to 50% of the tubing diameter, are locally provided at one or more predetermined points on at least one of two overlapping adjacent tube sections. When such tube sections are stacked together, adjacent return bends nest in these dimples, allowing the circuits to be more tightly packed than conventional non-dimpled return bends. An exemplary embodiment has dimples with a depth of between {fraction (1/16)}xe2x80x3 to {fraction (3/16)}xe2x80x3. However, dimpling is not limited to this. Actual dimpling size may be selected based on several criteria, including the desired degree of compression/density, structural considerations, and the maximum reduction in tubular cross-sectional area as allowed by fluid, gas or two phase velocity and/or pressure drop.
In an exemplary embodiment, dimpling is provided on both sides of every return bend. In an alternative embodiment, dimpling is provided on both sides of every other return bend, leaving adjacent return bends undimpled but producing the same overall effect. In yet another exemplary embodiment, each return bend is dimpled in two places on one side of the tubing so that regardless of the order of stacking of circuits, the tube bundles will always nest uniformly. In yet a further exemplary embodiment, dimpling can be performed on both sides of all tubes, but with a reduced or less pronounced dimple size. This will have the same net result as larger dimples being provided on only one side. In yet another embodiment, the same effect can be achieved by use of a non-circular reduced cross-section in the process direction. An example of this would be an elliptical cross-section.
In exemplary embodiments of the invention, the dimples can be formed en mass by a die or jig that forms the dimples substantially simultaneously to all required areas on a circuit. Alternatively, individual dimples can be formed during the formation of the serpentine return bends. The particular method of production may be selected based on the particular method of tube manufacture used.